Method and Apparatus for Pellicle Removal

ABSTRACT

A method and apparatus for removing a pellicle from a photomask wherein the adhesive between the pellicle frame and photomask is cooled sufficiently to allow the adhesive property of the adhesive to diminish to the point where the adhesive will release from the photomask with little or no mechanical force and leaving minimal adhesive on the photomask. The adhesive is cooled by way of manifolds containing coolant being brought in contact with the pellicle frame or by way of a coolant spray nozzles spraying coolant directly onto the pellicle frame.

TECHNICAL FIELD

This patent disclosure relates generally to photomask manufacture. Moreparticularly, the present invention relates to the removal of a pelliclefrom a photomask.

BACKGROUND

In the manufacturing of integrated circuit devices, photomasks,alternatively referred to as photoreticles or reticles, are used toproject patterns for the integrated circuits on a semiconductor wafer.To protect the photomask from contamination, which would impart errorsin the projected patterns, a pellicle comprised of a transparent polymerthin film contained within an aluminum or plastic frame can be affixedto one side of the photomask by an adhesive bond between the pellicleframe and the photomask.

During use of the pellicle it can become necessary to remove thepellicle for a number of reasons including repairing a defect detectedin the photomask, removing haze that has formed on the photomask underthe pellicle, replacing the pellicle due to mechanical damage orexposure damage. The method of removing the pellicle from the photomaskhas typically involved the application of mechanical force to thepellicle frame to separate it from the photomask. Problems encounteredwith mechanical separation, however, include incomplete adhesive removaland pellicle adhesive deposited in the pattern area of the photomask,referred to as stringers, both requiring time-consuming cleaning whichcan degrade the mask to the point of being unusable. Damages can alsooccur to the metal film on the photomask during mechanical separationrequiring costly repair to the photomask or require a new photomask tobe fabricated.

Alternative methods that have been used to remove the pellicle fromphotomasks include soaking the pelliclized photomask in a hot solvent todissolve the adhesive. These processes are not ideal because theyutilize flammable and/or toxic solvents. Another method that has beenused is placing the pelliclized photomask on a hot plate to melt theadhesive before mechanically separating the pellicle from the photomask.This process often leaves large amounts of pellicle adhesive on thephotomask requiring significant subsequent cleaning steps.

U.S. Pat. No. 4,255,216, entitled “Pellicle Ring Removal Method andTool,” describes a proposed solution to the problem of mechanicalpellicle removal that utilizes coolant to bring the adhesive for thepellicle below its embrittlement temperature while also causing theglass photomask to contract at a different rate from the contraction ofthe pellicle ring and adhesive thereby aiding in the separation of theadhesive from the photomask. In the proposed solution of the '216patent, the photomask is placed in the pellicle removal tool with thesecured facing down and the photomask is forced upwards away from thepellicle upon separation of the adhesive from the photomask. Coolant isapplied directly to the photomask on the opposite side of the pellicleand thus, the pellicle frame and adhesive are indirectly cooled bythermal transfer through the photomask. Because the quartz glass usedfor photomasks is generally selected for its temperature stability, thephotomask is not an ideal conductive medium for providing cooling to theadhesive and pellicle frame.

An improved process and apparatus for pellicle removal is needed tofurther reduce or obviate the need for harsh chemical cleaning ofphotomasks after pellicle removal, reduces the re-pelliclization cycletime, and reduces damage to photomasks that occur as a result of damageto the metal layer or deposition of pellicle adhesive in the maskpattern.

SUMMARY

In one aspect, the disclosure describes a method of removing a pelliclefrom a photomask. The method includes the steps of cooling the adhesivebetween a frame of the pellicle and the photomask. In this method, thestep of cooling the adhesive includes the steps of insulating a portionof the photomask and cooling a frame of the pellicle.

In another aspect, the disclosure describes an apparatus for separatinga pellicle from a photomask. The apparatus includes a first manifoldcontaining cooling fluid, wherein the first manifold is placed incontact with a first portion of a frame of the pellicle. The apparatusalso includes adjustable supports, wherein the adjustable supportssecure the first manifold relative to the pellicle frame and thephotomask.

In yet another aspect, the disclosure describes an apparatus forseparating a pellicle from a photomask. The apparatus includes a firstcoolant spray nozzle, wherein the first coolant spray nozzle is orientedto spray coolant onto a first portion of a frame of the pellicle. Theapparatus also includes adjustable supports, wherein the adjustablesupports secure the pellicle relative to the coolant spray nozzle. Theapparatus further includes a barrier, wherein the insulator is orientedto shield a portion of the photomask from the spray coolant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top perspective view of an exemplary apparatus for removalof a pellicle from a photomask in accordance with an aspect of thepresent disclosure.

FIG. 2 is a side perspective view of the apparatus of FIG. 1.

FIG. 3 is a partial cross-sectional side view of a cooling manifold ofFIG. 1 engaged with a photomask and pellicle.

FIG. 4 is a schematic representation of a coolant distribution system inaccordance with one aspect of the present disclosure.

FIG. 5 is a schematic representation of an alternate aspect of thepresent pellicle removal method and apparatus of the present disclosure.

DETAILED DESCRIPTION

Now referring to the drawings, wherein like reference numbers refer tolike elements, there is illustrated a pellicle removal apparatus 100 inaccordance with a first aspect of the present disclosure. The apparatus100 includes a base 102 supported by four support legs 104 onto whichvarious adjustable supports are provided. As depicted in this aspect ofthe disclosure, a first “L-shaped” manifold 106 is mounted on a sled 108so that it can be moved along the base in a first direction by rotationof a screw knob 110 connected to a threaded rod 112 provided in athreaded hole in the sled 108. The manifold 106 can be moved in a seconddirection, normal to the first direction, by rotation of a pinion gear114 that engages with a rack 116 affixed to the sled 108. A secondL-shaped manifold 118 is rigidly affixed to the base 102 so that thefirst manifold 106 and second manifold 118 form a rectangular enclosure.Support arms 106, 118 provide support for the corners of the L-shapedmanifolds and can be adjusted, respectively, by rotation of supports120, 122 affixed to threaded rods screwed into the base 102 to level themanifolds relative to each other.

Support plates 124, 126 are provided above the manifolds 106, 118parallel to two sides of the base 102 and can be moved toward and awayfrom the center of the base by operation of compression springs 130. Thecompression springs 130 are supported around shoulder screws 128 thatare connected to the support plates 124, 126 at one end and allowed toslide freely through the support stanchions 132. The support platesinclude engagement pieces 135 that can be moved upwards or downwardsrelative to the base by rotation of a knob 134 connected to a threadedrod (not shown) that screws into the engagement pieces. The supportplates 124, 126 are pressed outward away from the center of the base,compressing the springs 130 to allow loading the photomask into theapparatus.

A separate support stanchion 136 is affixed to the base 102 such thatthe stanchion can rotate around an axis normal to the plane of the base102. A v-grooved jig 138 is mounted to the rotatable support stanchion136 such that it can be moved upwards and downwards within a housing 140by rotation of a knob 142 connected to a threaded rod (not shown) thatis affixed to the v-grooved jig 138 in a manner that translatesrotational movement to lateral movement of the jig. The housing 140 canalso be moved outward and inward relative to the stanchion 136 byrotation of a knob 144 affixed to a threaded rod (not shown) that isscrewed into a threaded hole in the housing 140. The position of thehousing 140 can be fixed relative to the stanchion by means of athreaded locking knob 146.

Each manifold 106, 118 is provided with an intake 148 and exhaust 150supply line. As will be readily understood, the intake and exhaustsupply lines 148, 150 include various fittings and supply hoses, onlypart of which are shown in FIG. 1, to connect the pellicle removalapparatus 100 to a supply of coolant and the a reservoir for collectionof the coolant after passing through the manifolds 106, 118. In apreferred embodiment, liquid nitrogen is used as the coolant althoughany other suitable coolant can be used including argon, oxygen,hydrogen, helium, fluorine, methane, neon, and carbon monoxide. It isdesirable that the coolant utilized lower the temperature of theadhesive to or below the point where its adhesive quality is diminishedsufficient that it releases readily from the surface of the photomaskand remains on the surface of the pellicle frame. Portions of the intakeand exhaust supply lines have been removed from the view of the pellicleremoval apparatus 100 in FIG. 2. As shown in FIG. 2, the pellicleremoval apparatus includes a handle 152 that can be rotated to loosenthe stanchion 136 and allow it to rotate normal to the base 102 or canbe tightened to prevent rotational movement of the stanchion 136.Similarly, a handle 154 is included that can be rotated normal to thebase 102 to allow movement of the sled 108 or lock it in place.

As depicted in FIG. 3, a photomask 156 will be provided with a pelliclehaving a frame 158 that typically ranges from 3 millimeters in height to6.35 millimeters in height. Other typical pellicle frame heights are3.15, 4, 5 and 6 millimeters. The pellicle frame 158 is secured to thephotomask 156 by an adhesive 160. The construction and orientation ofthe manifold 106, 118 during operation is such that it can accommodatepellicle frames of any height. As shown in FIG. 3, the manifolds 106,118 include a thin wall section 162 that forms part of the enclosedcoolant fluid passage 164 of the manifolds 106, 118 and which is pressedagainst the pellicle frame 158 to allow thermal transfer cooling thepellicle frame and adhesive 160. In a preferred embodiment, themanifolds 106, 118 are 3D printed from titanium and the thin wallsection 162 of the manifolds 106, 118 have a thickness in the range of0.005 inches to 0.13 inches and preferably is 0.06 inches to allow thecreation of a notch 168 in at least one manifold 106. Titanium is usedin a preferred embodiment because it has a high heat transfer rate andlow coefficient of thermal expansion. The strength of titanium alsoallows reduction of the thickness of the thin wall section 162 toincrease the transfer of heat from the pellicle frame to the coolantwhile maintaining sufficient strength to prevent damage to the manifoldduring operation. In alternative embodiments, the manifolds areconstructed of a material having a low coefficient of expansion such asiron-nickel alloys or stainless steel. Other embodiments, that achievethe purposes of the present disclosure and reduce overall cost, includea combination of materials used in the manufacture of the manifolds 106,118 such as titanium for use in the thin wall section 162 and stainlesssteel for the balance of the manifold. In an alternative embodiment,weep holes are provided in the manifolds 106, 118 to allow the coolantto directly contact the pellicle frame to accelerate cooling.

In operation, the photomask 156 which is generally rectangular in shapeis secured on two sides by the support plates 124, 126 and in theopposite corner by the v-grooved jig 138. As will be readily understood,photomasks of various sizes can be held in the apparatus by adjustmentof the support plates 124, 126 and v-grooved jig 138 relative to thebase 102. In the implementation of this embodiment of the disclosure,the photomask 156 is held in the apparatus 100 with the pellicle facingdown and the engagement pieces 135 and v-grooved jig 138 are adjustedupwards or downwards to ensure that the manifolds 106, 118 do not touchthe photomask while also adjusting the engagement of the L-shapedmanifolds 106, 118 with the pellicle frame 158. In a preferredembodiment, faster release times of the pellicle frame 158 from thephotomask 156 is achieved by maximizing the contact between themanifolds 106, 118 with the pellicle frame 158. Pellicle frames 158 areprovided with vent holes with a filter (not shown) along the edge toallow pressure equalization in the cavity between the pellicle andphotomask. The notch 168 is provided in at least one manifold 106 thatis aligned with the filter of the pellicle during operation to allowpressure equalization to occur. In applications where it is notnecessary to provide a notch 168 aligned with a vent hole in thepellicle frame a thinner profile wall 162 of the manifold 106 isgenerally preferred to increase cooling capacity. In one embodiment,when the pellicle frame is released from the photomask by operation ofthe apparatus, the sled 108 will be retracted to release the grip on thepellicle and it will be allowed to fall away. In an alternateembodiment, the photomask can be moved away from the pellicle frame byraising the engagement pieces 135 and v-grooved jig 138.

An alternative embodiment is depicted in FIG. 4 in which a pellicle 170having a pellicle frame 172 that is mounted to a photomask 174 isdepicted. In this embodiment, the pellicle frame 172 is engaged on eachof the four sides by separate manifolds 176, 178, 189, 182. The use ofseparate manifolds allows the pellicle removal apparatus to accommodatea wider range of pellicle sizes. Coolant is supplied from an appropriatereservoir, such as a cryogenic storage dewar 166, through supply lines184, 186 to a first manifold 176. In this embodiment, coolant issupplied serially to each successive manifold 178, 180, 182 via supplylines 188, 190, 192 connecting the manifolds. It will be recognizedthat, in alternative embodiments, coolant can be provided in parallel tothe manifolds. Coolant flows from the last manifold 182 in the series toa collection reservoir 196 such as an open dewar. As depicted in FIG. 4,a valve is provided to control the supply of coolant to the manifolds.

As also depicted in FIG. 4, the pellicle removal apparatus is containedwithin a dew point controlled environment 200 such as a glove box,represented by dashed lines in FIG. 4, to prevent condensation fromforming during the process and to protect the photomask fromcontamination. In a preferred embodiment, the dew point controlledenvironment is nitrogen filled. Similarly, the pellicle removalapparatus 100 of FIG. 1 can also be provided in a dew point controlledenvironment. In alternative embodiments, the placement and securing ofthe photomask and pellicle frame in the pellicle removal apparatus ofFIG. 1 and FIG. 4, as well as the separation of the pellicle from thephotomask, is automated using robotic equipment contained in dew pointcontrolled enclosure 200.

In operation, a photomask with an attached pellicle is secured withinthe pellicle removal apparatus. In a preferred embodiment, a flow ofcoolant is supplied to the manifolds for sufficient time to allow theadhesive to release from the photomask leaving little or no residue. Inan alternate embodiment, the adhesive is cooled to an embrittlementtemperature at which the pellicle frame and adhesive can be detachedfrom the photomask by mechanical force leaving little or no residue.Separation of the pellicle frame and adhesive are accomplished as aresult of one or more of the of the following factors working alone orin combination: (1) the adhesive force of the adhesive being greatlydiminished by the coolant; (2) the different coefficient of thermalexpansion and contraction of the pellicle frame, adhesive and photomaskapplying a strain between the adhesive and the photomask; and (3)movement of the pellicle frame or photomask relative to the device, suchas by movement of the manifold relative to the pellicle frame, providinga strain force between the adhesive and photomask.

The methods of the present disclosure can be employed, for example, byreduction of the temperature of the adhesive to the point where theadhesive separates from the photomask with little or no strain beingapplied to the pellicle, as would be the case where the pellicle frameand photomask have similar coefficients of thermal expansion andcontraction. The methods of the present disclosure can also be employed,for example, by reducing the temperature of the adhesive to anembrittlement point where the adhesive would not separate from thephotomask but for the strain generated by the different coefficient ofthermal expansion and contraction of the pellicle frame, adhesive andphotomask. Because the methods and apparatus of the present disclosurecan be employed to utilize each of the foregoing factors alone or incombination, flexibility is provided in the selection of coolants,methods for the application of the coolants and the time required forthe pellicle removal.

In a preferred embodiment, the coolant is nitrogen and the manifoldcontaining the coolant is placed in contact with the pellicle frame fora period of three to five minutes. The amount of time during which thepellicle frame is cooled will be impacted by the type of coolant, themechanism employed for applying the coolant to the pellicle frame, andthe coefficient of thermal expansion of the particular pellicle,adhesive and photomask being operated upon and these can be readilydetermined for different applications by the use of the methods andapparatus of the present disclosure. In an exemplary photomask theacrylic adhesive will have a coefficient of thermal expansion andcontraction of approximately 75×10⁻⁶ m/m K°, an aluminum pellicle framewould have a coefficient of thermal expansion and contraction ofapproximately 22×10⁻⁶ m/m K°, and the quartz photomask would have acoefficient of thermal expansion and contraction of approximately0.77×10⁻⁶ m/m K°. As depicted in FIG. 3, in a preferred embodiment, themanifold 106, 118 contacts the pellicle frame 158 and adhesive 160 butdoes not come in contact with the photomask 156 thus protecting thephotomask from damage by the manifold. In addition, by spacing themanifold away from the photomask an insulating air-gap separationbetween the manifold and photomask is created, the pellicle frame coolsmore rapidly than the photomask providing a larger differential betweencontraction of the pellicle frame and adhesive from that of thephotomask and allowing the separation to occur more rapidly.

In an alternative embodiment, shown in FIG. 5, one or more spray nozzles202 can be used to spray coolant at the pellicle frame 204 and adhesive206. Although only shown on two sides, it should be understood thatspray nozzles would preferably be positioned to spray at all four sidesof the pellicle 204. An exemplary spray nozzle suited for thisapplication is the Eco-Snow Systems CO₂ spray nozzle 20-44220-001-02described in U.S. Pat. No. 6,173,916. In this embodiment, the Pellicleis placed against a vacuum chuck 208 that holds the pellicle during theremoval process. The vacuum chuck 208 reduces potential contamination ofthe photomask 210 if the pellicle is damaged during removal. Becausethis embodiment does not require mechanical securing of the pellicle inplace, exact registration of the pellicle in the apparatus is notcritical allowing the removal process to be carried out more rapidly. Insome embodiments, the gases utilized include CO2, N2,hydroflourocarbons, and R12 and other chloroflourocarbons. It will bereadily understood that the foregoing list of gases is only exemplaryand any other gas can be used as a coolant in this alternate embodimentthat achieves sufficiently low temperature during its expansion as itleaves the spray nozzle and impinges on the pellicle to achieve thedesired separation of the adhesive from the photomask. Moreover, byusing a vacuum chuck 208 that is as large as the largest pellicle, anysize and shape pellicle can be accommodated in this apparatus. Afterplacement of the pellicle on the vacuum chuck 208, the spray nozzles 202can be positioned relative to the pellicle frame 204 by adjustablesupports. It will be readily understood that positioning of the spraynozzles with respect to the pellicle can be by manual or automatedprocesses and equipment. In an alternative embodiment, an insulator orbarrier 212 can be inserted between the photomask 210 and the spraynozzle 202 to limit the cooling of the photomask 210.

Although described with particular spatial orientation of thepelliclized photomask with respect to the pellicle removal apparatus forpurposes of the embodiments of the foregoing description, it will bereadily understood that such orientation is not necessary in employingthe inventions described herein. The inventions described herein may beutilized in an apparatus in which the photomask is mounted with thepellicle facing upward, to the side, or in any desired spatialdirection. In addition, although described in the foregoing embodimentsas having particular structure to support and adjust the relativeplacement of coolant with respect to the pellicle and photomask, it willbe readily understood that other structure can be used to achieve thesame result without departing from the spirit and scope of the presentinventions.

It will also be appreciated that the foregoing description providesexamples of the disclosed system and technique. However, it iscontemplated that other implementations of the disclosure may differ indetail from the foregoing examples. All references to the disclosure orexamples thereof are intended to reference the particular example beingdiscussed at that point and are not intended to imply any limitation asto the scope of the disclosure more generally. All language ofdistinction and disparagement with respect to certain features isintended to indicate a lack of preference for those features, but not toexclude such from the scope of the disclosure entirely unless otherwiseindicated.

Recitation of ranges of values herein are merely intended to serve as ashorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context.

We claim:
 1. A method of removing a pellicle from a photomask,comprising the steps of: cooling the adhesive between a frame of thepellicle and the photomask; wherein the step of cooling the adhesivecomprises the steps of: insulating a portion of the photomask; cooling aframe of the pellicle.
 2. The method of claim 1, wherein the step ofcooling the frame of the pellicle comprises placing a manifoldcontaining coolant against the pellicle frame.
 3. The method of claim 2,wherein the manifold is placed against the pellicle frame for a periodof three to five minutes.
 4. The method of claim 2, wherein the step ofinsulating a portion of the photomask comprises spacing the manifoldfrom the photomask to provide an air-gap between the manifold andphotomask.
 5. The method of claim 4, further comprising the step ofallowing the adhesive to release from the photomask.
 6. The method ofclaim 4, further comprising the step of mechanically separating theadhesive from the photomask.
 7. The method of claim 1, wherein the stepof cooling the frame of the pellicle comprises spraying a coolant ontothe frame of the pellicle.
 8. The method of claim 7, wherein the step ofinsulating a portion of the photomask comprises inserting a barrierbetween the coolant spray and the photomask.
 9. The method of claim 8,further comprising the step of allowing the adhesive to release from thephotomask.
 10. The method of claim 8, further comprising the step ofmechanically separating the adhesive from the photomask.
 11. Anapparatus for separating a pellicle from a photomask, comprising: afirst manifold containing cooling fluid, wherein the first manifold isplaced in contact with a first portion of a frame of the pellicle; andadjustable supports, wherein the adjustable supports secure the firstmanifold relative to the pellicle frame and the photomask.
 12. Theapparatus of claim 11, wherein the first manifold is titanium and has athin wall section having a thickness in the range of 0.005 to 0.13inches.
 13. The apparatus of claim 11, wherein the first manifold istitanium and has a thin wall section having a thickness of 0.06 inches.14. The apparatus of claim 11 further comprising structure for securingthe photomask in the apparatus.
 15. The apparatus of claim 14, furthercomprising a dew point controlled enclosure in which the first manifoldand adjustable supports are contained.
 16. The apparatus of claim 15,further comprising a second manifold, wherein the second manifold isplaced in contact with a second portion of the frame of the pellicle.17. An apparatus for separating a pellicle from a photomask, comprising:a first coolant spray nozzle, wherein the first coolant spray nozzle isoriented to spray coolant onto a first portion of a frame of thepellicle; an adjustable support, wherein the adjustable support securesthe pellicle relative to the coolant spray nozzle; and a barrier,wherein the barrier is oriented to shield a portion of the photomaskfrom the spray coolant.
 18. The apparatus of claim 17, wherein theadjustable support comprises a vacuum chuck.
 19. The apparatus of claim18, further comprising a dew point controlled enclosure in which thefirst coolant spray nozzle, adjustable supports, barrier and vacuumchuck are contained.
 20. The apparatus of claim 19, further comprising asecond coolant spray nozzle, wherein the second coolant spray nozzle isoriented to spray coolant onto a second portion of a frame of thepellicle.